1. Field of the Invention
The present invention relates to an image forming apparatus that has a photosensitive member exposed to plural beams, and a control apparatus for a light source of the image forming apparatus.
2. Description of the Related Art
In electrophotographic type image forming apparatuses, a laser light beam is irradiated onto a uniformly charged photosensitive member to form an electrostatic latent image thereon, the electrostatic latent image is developed to form a toner image on the photosensitive member, and the toner image is transferred and fixed to a recording medium for image formation on the recording medium.
Some of such image forming apparatuses have a rotary polygonal mirror having plural reflection surfaces, and cause plural laser light beams to enter the same reflection surface of the rotary polygonal mirror, and scan the photosensitive member with the light beams deflected by the reflection surface and passing through lenses such as fθ lenses. Hereinafter, a scanning direction of the light beams on the photosensitive member will be referred to as the main scanning direction.
In such an image forming apparatus, the photosensitive member is exposed with the plural laser light beams (deflected by one reflection surface of the rotary polygonal mirror) at predetermined intervals in a rotating direction of the photosensitive member, i.e., in a sub-scanning direction. Accordingly, plural scanning lines can be formed on the photosensitive member during one scan cycle, whereby image formation can be performed at high speed.
However, there is a case where the reflection surfaces of the rotary polygonal mirror have slightly different angles relative to a rotation axis of the mirror, and optical paths of light beam reflected by different reflection surfaces become different from one another due to differences between the reflection surface angles. In that case, an interval between upstream-most one of exposure positions of the light beams (deflected by one reflection surface of the rotary polygonal mirror) on the photosensitive member in the rotating direction of the photosensitive member and downstream-most one of exposure positions of the light beams (deflected by the next reflection surface of the mirror) in the rotating direction of the photosensitive member does not become equal to an interval between adjacent ones of exposure positions of plural light beams deflected by one reflection surface of the mirror.
Due to unevenness of the interval between exposure positions of light beams, density unevenness occurs in a toner image in the rotating direction of the photosensitive member. Thus, there has been disclosed an image forming apparatus that controls light amounts of light beams on a per reflection surface basis to thereby prevent density unevenness (see, for example, Japanese Laid-open Patent Publication No. 2008-116664).
However, exposure positions of light beams vary under influence of characteristics of lenses such as fθ lenses disposed on optical paths between the rotary polygonal mirror and the photosensitive member.
FIG. 1 shows a result of measurement of exposure positions of plural light beams.
In the measurement to obtain the illustrated measurement result, an image forming apparatus was used that is configured such that images of first to sixteenth laser light beams deflected by one of reflection surfaces of a rotary polygonal mirror are formed on a surface of a photosensitive member at intervals corresponding to resolution of 2400 dpi, and exposure positions of the first to sixteenth laser light beams were measured by using an array type CCD sensor.
In FIG. 1, black circles represent the exposure positions of the first to sixteenth light beams on the center and both ends of the photosensitive member (main scan image heights, i.e., positions on a surface of the photosensitive member where laser light images are formed). It should be noted that illustrations of exposure positions of the light beams on the remaining portions of the photosensitive member are omitted. A fine curved line extending horizontally at an upper part of FIG. 1 represents a scanning line (scanning locus) of the first light beam on the photosensitive member, fine straight lines extending horizontally at a central part of FIG. 1 represent scanning lines of the eighth and ninth light beams, and a fine curved line extending horizontally at a lower part of FIG. 1 represents a scanning line of the sixteenth light beam. It should be noted that illustrations of scanning lines of the second to seventh light beams and those of the tenth to fifteenth light beams are omitted.
It is preferable that intervals between exposure positions of adjacent light beams at respective positions in the main scanning direction be uniform. However, since the incident position to lenses is different between respective light beams, lens aberrations at respective incident positions are slightly different from one another. As a result, the scanning lines are curved as shown in FIG. 1, and intervals between light beams at each end portion of the photosensitive member in the main scanning direction become smaller than intervals between light beams at a central portion of the photosensitive member in the main scanning direction. Generally, each lens has a higher optical performance at parts closer to its optical axis. In other words, scanning lines of light beams entering at positions of the lens remoter from the optical axis (i.e., scanning lines of the first and sixteenth light beams in the example of FIG. 1) are more noticeably curved. It should be noted that the scanning lines can be curved in a direction opposite from the curved direction shown in FIG. 1 depending on lens characteristics. If intervals between scanning lines (exposure intervals) vary depending on the position in the main scanning direction, density unevenness occurs in a toner image.